[Aavso-photometry] transformations

[RICHARD MILES]

Arne,

I know your comments are largely directed at all-sky catalogues but can you 
say something about the CMC-14 catalogue?

Roger Dymock, myself and John Greaves (yes, John, we've included your name 
as you drew our attention to the possibility) are co-authors of a paper 
which Roger and I have submitted to the BAA Journal on the subject of using 
CMC-14/2MASS data to generate V magnitudes for stars.  You can also use 
other conversion formulae to generate Rc magnitudes and Sloan-r* mags.  The 
catalogue has virtually 100% coverage from -30 to +50 Dec  (apart from a gap 
south of -15 Dec between 5.5-10.5 hours in RA), and is photometrically 
useful from about 9th mag down to about 15th mag.  BTW: I am currently 
helping in the development of software to derive automatic plate solutions 
for V and R based on CMC-14 data, which may be useful for VS observers 
working in these passbands.  Note that it is primarily intended for 
observers working filtered or unfiltered on asteroid photometry.  Hopefully 
the facility should be available shortly.

More anon.
Richard Miles

----- Original Message ----- 
From: "arne" <arne at aavso.org>
To: "Aavso-Photometry" <aavso-photometry at mira.aavso.org>
Sent: Monday, July 14, 2008 5:37 PM
Subject: Re: [Aavso-photometry] transformations


> Steven Orlando wrote:
>> Arne,
>>
>> You mentioned catalogues, which current catalog is the best for
>> photometry? Which catalogues currently being developed will the best for
>> photometry?
>>
>> I know that for some catalogues, like Tycho-2 and 2-MASS, there are
>> conversion formulas to convert to the Standard BVRI system.
>>
> There is no perfect catalog out there; all are compromises when it
> comes to photometry.  You pick one from column A, the next from column B.
> The two all-sky catalogs that are best are Tycho2 (basically B&V down
> to perhaps 10th magnitude) and 2MASS (JHK, all sky).  All professionals
> use 2MASS for their NIR photometry, and its great.  For optical work,
> though, it is much harder.  I'm using a dozen or so catalogs for
> the comparison star database, with logic to select between them
> based on their limitations.  We hope to rectify that situation in
> a few months with the Ellijay survey.  There are other future surveys
> underway, or nearly so, such as PanStarrs and SkyMapper, with the 
> situation
> improving in a few years.
>
> So if you are currently looking for a V magnitude of a star, I'd:
> (1) use Tycho2 and conversion formulae if the star is brighter than 10th
> (2) use SDSS and conversion formulae if the star is fainter than 14th
> (3) use ASAS in between
>
> Note that the latter two catalogs do not cover the entire sky,
> and have other limitations, so this simple solution will fail quite often.
> Arne